This invention relates to a process for the production of low-needle silicon nitride of high .alpha.-content, to a silicon-, nitrogen- and phosphorus-containing compound as starting material and to a process for the production of this compound.
Crystalline silicon nitride powder for engineering ceramic applications is generally obtained by thermal decomposition of silicon diimide or by crystallization of amorphous silicon nitride at approx. 1450.degree. C. The calcination product generally contains more than 80% .alpha.-silicon nitride. The rest consists of .beta.-silicon nitride and amorphous fractions, depending on the halogen and oxygen content of the starting materials and on the temperature.
Silicon nitride powder produced in this way generally has a high percentage content of large (&gt;2 .mu.m) acicular or columnar crystals which largely consist of the .beta.-phase. This is a disadvantage when the powder is used for the production of sintered compacts because a low green density is obtained with a high percentage content of acicular crystals, resulting in a low density of the sintered workpiece. Accordingly, sintered compacts of the type in question show poor strength and fracture toughness values on account of their high residual porosity.
Accordingly, Si.sub.3 N.sub.4 powder used for engineering ceramics should have an .alpha.-content of at least 90% with very few, if any, needles. Spherical particles less than 1 .mu.m in size are the most favorable.
U.S. Pat. No. 4,770,830 describes a process for the production of needle-free Si.sub.3 N.sub.4 having an .alpha.-content of &gt;90%. In this process, a small quantity of crystalline silicon nitride (approx. 5%) is added to the amorphous starting product. The crystalline powder added must have an .alpha.-content of greater than 90% and a high percentage content of particularly fine powder. The percentage contribution of the particles smaller than 0.05 .mu.m in size to the total weight of additive and product to be calcined must amount to at least 0.1% by weight. In addition, the heating rate between 1200.degree. and 1350.degree. C. should be at least 15.degree. C./minute.
The .alpha.-contents and particle sizes obtainable by this process are described as satisfactory. However, the addition of .alpha.-Si.sub.3 N.sub.4 crystallization nuclei does not reduce the intragranular oxygen or halogen content of the starting powder. This is because powders produced by the diimide process still contain percentage of intragranular oxygen which promotes needle formation and hence has an adverse effect on the sintering properties.
By contrast, Si.sub.3 N.sub.4 powders produced by a gas-phase process still contain chemically bound chloride which is also said to have an adverse effect on the sintering properties and strength of the resulting moldings.